1. Field of the Invention
The present invention relates generally to testing telecommunications networks, and particularly to detecting dropped packets over a packet switched network.
2. Technical Background
New telecommunications technologies are emerging that employ packet switching instead of, or in addition to, the traditional circuit-switched technologies provided by the public switched telephone networks (PSTN). Providing full duplex voice over the Internet (VoIP) is of particular importance. VoIP services are attractive to commercial long-distance carriers because they enable the use of global Internet transport facilities to carry traffic that is presently being carried over dedicated circuit-switched facilities. The potential benefits are enormous, considering the possibilities for reduction in data exchange capacity required to support a telephone call, opportunities for developing new features and functions for voice services, and the economies of scale from the use of one kind of transport for all telecommunications services. However, a major impediment to the use of VoIP services is concern as to the likely user perception of the quality of full-duplex voice communications using VoIP.
Telephone connections have always been subject to impairments in the form of noise, attenuation, distortion, cross-talk and echo. Such impairments are particularly common to analog portions of the network, such as subscriber loops and frequency domain multiplexing equipment. Digital transmission alleviates many of these problems but also introduces quantization noise and distortions due to bit errors in the digital signal. Even with perfect digital transmission applied to long haul transmissions, a typical telephone connection includes many analog components where impairments can occur. As a result, a poor connection or malfunctioning piece of equipment can produce unacceptably poor reproduction of voice, or conditions on a connection that customers will find objectionable or intolerable. When there is a high incidence of poor connections, the customer may complain to the service provider or to a regulatory authority, or simply change long distance carriers. Thus, perceived quality of telephone connections is a major factor affecting the reputation and marketability of long distance telephone services.
To guard against poor quality, service providers have developed methods to obtain objective quality measurements for a line, piece of equipment, or an end-to-end telephone connection. These measurements can help the service provider detect and gauge impairments, pinpoint weak elements, and correct deficiencies that degrade user perception of quality. As a result, the average consumer has to come to expect a certain quality of service from the PSTN.
With the proliferation of voice-over-packet technologies, maintaining quality of service comparable to the PSTN is a major concern of service providers, equipment vendors, and ultimately the consumers of telecommunications services. Unlike circuit switched traffic, real time voice transmission using packet switched technologies is sensitive to the packet loss, packet delay, and jitter that are characteristic of packet switched networks. In particular, packet loss and packet delay variations may result in transmissions missing some packets, thereby creating an audio signal that is distorted, garbled, or otherwise degraded.
IntServ and Diffserv protocols have been proposed for improving the reliability and consistency of packet transport for packets carrying voice. RTCP (Real Time Transmission Control Protocol) is being deployed to enable real-time measurements of the receipt of packet data, and for reporting the measurements to a sender or to a network quality monitoring location. However, in a hybrid network that includes both a circuit switched network, such as the PSTN and a packet switched network, the detection of dropped packets is problematic, because none of the protocol data is relayed to the end station. The problem, then, is how to measure dropped frame rates codec-to-codec across a packet switched network when there is no access to packet transmission control data. This problem is exacerbated by the fact that dropped frame rates may be manifested in two distinct ways. First, the dropped packet may simply be skipped. If so, the result is a null (a segment of time with no signal energy at all) in the received voice signal. Alternatively, when a packet has not been received in time to maintain a continuous flow of voice, the receiving codec may simply insert the previous packet or a segment of white noise to avoid a null. This packet loss concealment routine results in repetition of speech waveforms or other phenomena that distort speech signals. It also has the effect of disguising the fact that a dropped packet that would have otherwise been revealed by a null was lost. Thus, the presence of nulls will indicate dropped packets, when no packet concealment routine is applied, or the number of dropped packets exceeds the dynamic range of the operant packet loss concealment routine. However, the dropped packets whose occurrence is disguised by a packet loss concealment routine will be more difficult to detect mechanically.
What is needed is a system and method for detecting dropped packets across a packet switched network when there is no access to packet network transmission control data that reliably detects dropped packets, absent any knowledge of whether, or what kind of, a packet loss concealment technique is being used.